A method for energy efficient and fast rotary drilling in inhomogeneous and/or hard rock formations

ABSTRACT

It is described a rotary drilling system ( 10 ) for drilling a borehole ( 12 ) in inhomogeneous and/or hard rock formations. The system ( 10 ) comprising: a rotary drill bit ( 20; 50 ), and at least two electrodes ( 21, 22, 23, 51, 52, 53 ) arranged for pulsing between said electrodes of at least one high voltage electro pulse ( 41 ), wherein said at least one high voltage electro pulse ( 41 ) is generated in response to a detected resistance on the drill bit ( 20 ) from a rock formation ( 40 ) in front of the drill bit ( 20 ) while rotating, in order to create small/micro-cracks ( 45 ) in the rock formation ( 40 ).

INTRODUCTION

The present invention concerns a rotary drilling system and a method fordrilling a borehole in inhomogeneous and/or hard rock formations.

BACKGROUND OF THE INVENTION

Rotary drilling is generally used today for drilling of deep wells fore.g. oil, gas, mining and geothermal energy exploration and excavation.In the resent year, PDC (Polycrystalline Diamond Compact) bits have beenthe fastest developing drilling bit technology, delivered in a largevariety of different shapes and sizes, mainly optimized to rock typesand well diameter size. Whenever possible it is the preferred bit choicefor any drilling. For high temperature and/or high pressure environmentsthey also have a clear benefit to many other drill bit concepts due tono moving parts. However, a major disadvantage by PDC bits is theirinability to drill efficiently in inhomogeneous (soft-hard) rockformations, due to large wear from dysfunctional vibrations (bit impactwear) and abrasive wear. Due to similar stability issues, they haveneither been suited for large well diameters. When drilling in rockformations, the friction between the drill bit under rotation and therock formation will vary also depending on the rock properties, e.g.hardness, porosity etc. This friction is experienced as a resistance onthe drill bit by the rock formation. Drilling in inhomogeneous or hardrock formation results in highly varying resistances experienced by thedrill bit during drilling. A PDC bit tends to create stick-slip, causedwhen cutters teeth are stalling in hard rock due to excessive depth ofcut when coming from a softer to a harder rock formation. The consequentvibrations are potentially severe when the drill string has accumulatedenough torque to break the cutters loose and the drill string up-winds.Stick-slip is the root cause of many costly and time consuming problemsin drilling operations; vibration related equipment failure, drillstring failure, bit impact damage and slow rate of penetration (ROP).These technical obstacles are being progressively addressed, e.g. by newbit designs, new hybrid bit designs, shock-absorbers above the bit(mechanical decrease of bit torque), or addition of a torsional impacthammer function to the bit in order to provide additional torsionalenergy to assist in fracturing the formation. Similar problems may moreor less also be experienced by other rotary drill bits as e.g. rollercone bits, hybrid roller cone—PDC bits, and cutter disks in rotarytunnel excavators. Drilling by an electro pulse boring (EPB) method iswell known and has been described by e.g. V. F. Vajor et. al. in“Physics Vol. 4” of Tomsk Polytechnic University (Russia) 1996.Different solutions for EPB drilling methods and equipment are given inU.S. Pat. No. 7,784,563, U.S. Pat. No. 7,530,460 and U.S. Pat. No.8,109,345, including combinations of electrodes for electro pulsing withmechanical cutters on the drill-head. However, in the mentioned patents,the electro pulse boring (EPB) is set to be the main excavation methodthroughout the drilling processes, either by general fracturing (makingcutter bits) of the rock in front of the drill head or by fracturing thesurrounding rock material to facilitate drilling in the direction of thedirected electric energy.

SUMMARY OF THE INVENTION

The invention solves or at least alleviates the problems by the priorart drilling systems.

In a first aspect, the present invention relates to a rotary drillingsystem for drilling a borehole in inhomogeneous and/or hard rockformations. The system comprises a rotary drill bit. Furthermore, thesystem comprises at least two electrodes. Said electrodes are arrangedfor pulsing therebetween of at least one high voltage electro pulse.Said at least one high voltage electro pulse is generated in response toa detected resistance on the drill bit from a rock formation in front ofthe drill bit while rotating, in order to create small/micro-cracks inthe rock formation.

The electrodes may be incorporated in the rotary drill bit. The at leastone high voltage electro pulse may be generated when the detectedresistance of the drill bit exceeds a predetermined limit under rotarydrilling. The predetermined resistance limit under rotation isdetermined based on a number of parameters e.g. the hardness of the rockformation, the drilling equipment and the drill bit, and may vary whenpre-detected devices are used. The rotary drilling system may furthercomprise at least one sensor arranged in the drill bit for detecting theresistance on the drill bit from the rock formation. Alternatively, atleast one sensor may be arranged in the drilling system for detectingthe resistance on the drill bit from the rock formation. In a furtherembodiment, the rotary drilling system may further comprise a radar orsonar for pre-detecting the resistance on the drill bit from the rockformation.

Further, an electro pulse device may be connected to said electrodes andadapted for generating said at least one high voltage electro pulse. Asensing device may be arranged for detecting the resistance to a drillbit from the rock formation in front of the drill bit under rotarydrilling. The system may include an electronic switch for electropulsing. The system may further include at least one of: a capacitor, arectifier and a transformer of low to high voltage and/or current. Thesystem may further include an electro pulse device for controlling andgeneration of the high voltage electrode pulses. The electronic switchmay form part of the electro pulse device. The electro pulse device mayfurther comprise at least one of: a capacitor, a rectifier and atransformer of low to high voltage and/or current.

The rotary drilling system may further comprise an electrical sourcecomprising at least one of: a battery pack or batteries, a mud drivengenerator, water driven generator, a compressed air generator, a rotarydrill string driven generator, and cables to surface. The rotarydrilling system may comprise a drilling assembly. The rotary drill bitmay be arranged in the drilling assembly. In an embodiment, the electropulse device may be arranged in the drilling assembly. The drill bit mayfurther comprise at least one of: teeth, scrapers and cutter disks, andwherein said at least two electrodes are incorporated in at least oneof: the drill bit itself, the teeth, the scrapers and the cutter disks.

The system may be adapted for tunneling, and the drill bit may then be arotary tunnel excavator.

In a further aspect, the invention provides a rotary drilling method fordrilling a borehole in inhomogeneous and/or hard rock formations. Themethod comprising rotary drilling of a borehole using a rotary drillbit, pulsing between at least two electrodes of at least one highvoltage electro pulse, wherein said at least one high voltage electropulse is generated in response to a resistance on the drill bit from arock formation in front of the drill bit while rotating, creatingsmall/micro-cracks in the rock formation.

The at least two electrodes may be incorporated in the drill bit. Themethod may further comprise generating said at least one high voltageelectro pulse when the detected resistance on the drill bit exceeds apredetermined limit under rotary drilling. The detected resistance maybe in the form of a detected torque on the drill bit from the rockformation. Detection of the resistance may be pre-detected using asuited detecting device in the drill bit or drilling assembly. Thegeneration of said at least one high voltage electro pulse may beperformed or initiated when the detected resistance on the drill bitexceeds a predetermined limit under rotary drilling, or by a signal fromthe device pre-detecting resistance. The predetermined resistance limitor torque limit under rotary drilling may be determined based on anumber for factors, e.g. rock formation hardness, the equipment anddrill bit used, and may vary by pre-detecting the resistance from rockformation. Said at least one high voltage electro pulse may have a valueof about 100 kV or more.

The method may further comprise excavating the small/micro-cracked rocklattice or matrix. The method may further be adapted for tunneling, andthe drill bit may then be a rotary tunnel excavator.

In general, the present invention provides a technology system that“softens up” the rock in front of a rotary drill bit by formation ofsmall/micro-cracks in the rock lattice when the resistance on the drillbit or drill bit teeth/cutters exceeds, or are to exceed, a given limitwhile rotary drilling. The rock in front of the rotary drill bit is“softened up” by one or more short-duration, high voltage electropulse(s) given through separate electrodes in the rotary drill bit,causing small/micro-cracks in the rock formation between and/or aroundthe electrodes. The small/micro-cracks in the rock lattice will avoidstick slip, high torque and abrasive wear of any rotary drill bit, whilekeeping an even and high rate of penetration. By “softening up” the rockformation with the high voltage electro pulse(s) when needed, the drillbit is experiencing almost the same resistance from the rock formationall the time during drilling, regardless of the properties of the rockformation. This enables maintaining a constant or little varyingrotational speed of the drill bit resulting. This results in anefficient drilling process also reducing risks for damage on drillingequipment. The “soften up” of the rock formation in front of the rotarydrill bit in the present invention may only be activated when neededbased on the detected resistance on the drill bit. The electro pulsingin the present invention is only used for forming small/micro-cracks inthe rock formation in front of the drill bit and not for cracking of therock formation for rock bits. Cracking of the rock formation for rockbits are used when drilling with an electro pulse method as the maindrilling method. The present invention, which activates the high voltageelectro pulse(s) only when needed based on the detected resistance onthe drill bit, keeps the total energy demand to a fraction of what isusually needed for either the rotary drilling bit technology or electropulsing drilling used separately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a rotary drilling system according to an embodimentof the present invention,

FIG. 2-4 are showing steps of the principle for “softening up” the rockin front of a rotary drill bit by small cracks/micro-cracks according tothe present invention,

FIG. 5 illustrates another rotary drilling system according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

In the following description, details are set forth to provide anunderstanding of the present invention. However, it will be understoodby those of ordinary skill in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiment may be possible.

The present invention provides a system and a method for softening upthe rock formation in front of a rotary drill bit by creation ofsmall/micro-cracks in the rock lattice or matrix when the resistance onthe drill bit is to, or exceeds a predetermined limit while rotarydrilling. This enables efficient drilling with approximately constantrotational speed on the drill bit as the resistance experienced by thedrill bit due to friction between the rotating drill bit and the rockformation is maintained about a same level throughout the drillingprocess.

A rotary drilling system 10 according to an embodiment of the presentinvention, is shown in FIG. 1. The system 10 comprises a drillingassembly 11 used to form a borehole 12, e.g. a wellbore. The wellboremay be drilled in any direction but usually, but not restricted thereto,from a surface into any desired formation. The drilling assembly 11 maybe based on any standard rotary drilling technology. The rotary drillingassembly 11 and the rotary drilling system 10 may be selected from anyknown rotary drilling system and support systems including any knownequipment, methods, and procedures known to anyone skilled in the art ofrotary drilling.

In front of the drilling assembly 11 there is a rotary drill bit 20. Atleast two electrodes are arranged for pulsing between said electrodes ofat least one high voltage electro pulse (41). The at least twoelectrodes (21, 22, 23) may be incorporated in the rotary drill bit(20). The body of rotary drill bit (20) may also in itself be anelectrode. The at least one high voltage electro pulse (41) (illustratedin FIG. 2) is generated in response to a detected resistance on thedrill bit (20) from a rock formation (40) in front of the drill bit (20)while rotating. The at least one high voltage electro pulse (41) createssmall/micro-cracks (45) in the rock formation (40). The at least onehigh voltage electro pulse (41) is generated when the detectedresistance on the drill bit (20) under rotary drilling exceeds apredetermined limit. The predetermined resistance limit under rotationis determined based on a number of parameters e.g. the hardness of therock formation to be drilled, the drilling equipment used and the drillbit used.

The rotary drill bit 20 may be provided with drill bit teeth 21 and/orscrapers 22 for excavation of the rock formation in front of the drillbit 20. The bit teeth 21 or scrapers 22 on the drill bit 20 may also inan embodiment include the at least one electrodes 21, 22 for highvoltage electro pulsing. The drill bit 20 may also include separateelectrodes 23 or be an electrode in itself. The electrodes 21, 22 and/or23 may be of any suited material and placed at any suited place on thedrill bit 20 in order to efficiently “soften up” (createsmall/micro-cracks in) the rock in front of the drill bit 20 by pulsingof high voltage current between the at least two electrodes 21, 22, 23and/or the drill bit itself. The voltage used for the pulsing betweensaid at least two electrodes 21, 22, 23 and/or the drill bit itself, maybe of any high voltage. The at least one high voltage electro pulse mayhave a value of about 100 kV or more. Drill bit 20 may contain anydevice for injection of insulating drilling fluid for the electropulsing, if needed. Other drill-bits, equipped with or without thepresent invention with drill-bit 20 in assembly 11 may be used in otherplaces along the drilling assembly system 10, as e.g. reamers forwidening the borehole 12.

Electrical connections to the electrodes 21, 22, 23, and or the drillbit 20 itself, may be incorporated in the drill bit 20 in any suitableway. The electrical connections may further be connected to an electricswitch 35 in the drilling assembly 11 by any suitable means. Theelectric switch 35 for electro pulsing may be of any kind e.g. anelectronic switch or a mechanical switch, generating high voltageelectric pulses at given value and frequency. The switch 35 will only beoperating when needed for the drilling purposes. The switch 35 may beengaged or triggered in response to the measured or pre-detectedresistance on the drill bit (20) from a rock formation (40), from theresistance sensing device 30 or detecting device 31. The resistance onthe drill bit (20) from a rock formation (40) may be detected by aresistance sensing device (30), or pre-detected by a detecting device(31). The resistance sensing device 30 measures the resistance (force)on the drill bit 20 from the rock formation in front of the drill bit20, while the drill bit 20 is rotating. The resistance sensing device 30may e.g. be in the form of a mechanical (e.g. spring compression),electronic or any combinations thereof sensor(s) arranged in the drillbit itself, or as a mechanical or electronic torque sensing devicedetecting the torque on the drill bit 20 arranged in the drillingassembly 11. The resistance sensing device 30 may be arranged on thedrill bit 20, directly behind the drill bit 20, or in other places onthe drilling assembly 11, or rotary drilling system 10. The resistancesensing device 30 may also be of any other suitable kind. When theresistance sensing device 30 detects a resistance experienced by thedrill bit 20 due to the rock formation in front of the drill bit,exceeding a the predetermined value/limit, the switch 35 is triggered orengaged, and the electro pulsing between the electrodes 21, 22, 23, andor the drill bit 20 itself, starts. The at least one high voltageelectro pulse 40 is generated in response to the detected torque on thedrill bit 20.

Resistance to drill bit 20 may alternatively, or in parallel toresistance device 30, be pre-detected by a detecting device 31. Whenpre-detecting resistance from the rock formation 40 in front of thedrill bit 20 exceeds a predetermined value/limit, electro pulses 41between the electrodes 21, 22, 23 and/or the drill bit 20 itself, may beengaged by electronic switch 35 shortly before or simultaneously as therotary drill bit 20 engage harder rock formation. The detecting device31 may be of any suited kind (e.g. radar or sonar), and may detect rockproperties in front of the drill bit 20 by any suited means, e.g. byacoustic or electronic signals or by electric resistance betweenelectrodes 21, 22 and/or 23, and may be integrated in drill bit 20,directly behind the drill bit 20, or in any other places on the drillingassembly 11.

The resistance sensing device 30 and/or detecting device 31, and theelectronic switch 35 may be arranged together as an integrated unit.

The high voltage electro pulses supplied to the electrodes 21, 22, 23and or the drill bit 20 itself, through switch 35 may e.g. be energizedfrom a capacitor(s) 36 in the drilling assembly 11. Energy may besupplied to the capacitor(s) 36 by an internal energy supply in thedrilling assembly 11 or trough electric cables from the surface of theborehole 12. The drilling assembly 11 may also include a rectifier ortransformer of low to high voltage and/or current. The internal energysupply may be provided by an electrical source 37, including but notlimited to e.g. batteries or a battery pack. The batteries/battery pack37 may also include or be substituted by e.g. any of a mud drivengenerator, a water driven generator, a compressed air generator, or arotary drill string generator.

In an embodiment a high voltage electro pulse device may comprise theswitch 35 for controlling and/or initiating the high voltage electropulsing. The high voltage electro pulse device may further comprise atleast one of: the capacitor 36, rectifier and transformer of low to highvoltage and/or current. All or some parts of the high voltage electropulse device can be arranged in the drilling assembly 11.

In principle, the electro pulse devices 21, 22, 23, 30, 31, 35, 36, and37 in the drilling assembly 11 of the present invention operateindependently of any other parts or operations of the rotary drillingsystem 10 when needed for efficient drilling. In any failure of theelectro pulse functions in the system of the present invention, theremaining rotary drilling system of the drilling assembly system 10 willperform drilling as efficient as an equal rotary drilling system withoutthe present invention.

The principle of “soften up” the rock in front of a rotary drill bit bysmall/micro-cracks in the rock lattice when the torque of the drill bitteeth/cutters, detected by sensing device 30, exceeds a givenlimit/value while drilling are illustrated in FIG. 2, FIG. 3, and FIG.4. Reference numbers referred to below, but not shown in FIG. 2, FIG. 3,and FIG. 4, are shown in FIG. 1. FIG. 2 illustrates that the cutterteeth 21 or scrapers 22 of the drill bit 20 hit a hard rock formation40, resulting in a resistance experienced by the cutter teeth 21 orscrapers 22 from the rock formation that exceeds the limit for thesensing device 30. The sensing device 30, thus engages the switch 35.One or more short high voltage electro pulse(s) 41 is/are then generatedthrough the rock matrix 40 as illustrated in FIG. 2. After the electropulse(s), the lattice of the rock formation 40 will contain internalsmall/micro-cracks 45 as illustrated in FIG. 3. The force from the teeth21 or scrapers 22 in the drill bit 20 that is now required to excavatethe rock matrix 40, which now contain small/micro-cracks 45, will beless than the predetermined limit set for the sensing device 30. Thesmall/micro-cracked rock lattice 40 will then be excavated as shown inFIG. 4. When the rock lattice with small/micro-cracks has beenexcavated, the electro pulse process may be reinitiated as the teeth 21or scrapers 22 of the drill bit 20 again hit hard rock formation 40resulting in a detected resistance above the predetermined value for theformation to be drilled, as illustrated in FIG. 2. During drilling, highvoltage electro pulses will be generated when resistance values abovethe predetermined limit are sensed by the sensors or sensing device 30.This enables efficient drilling with approximately constant rotationalspeed on the drill bit as the resistance experienced by the drill bitdue to friction between the rotating drill bit and the rock formation ismaintained about a same level throughout the drilling process. How oftenthe high voltage electro pulses are generated, depends e.g. on thepredetermined limit set and the properties of the rock formation.

The principle of “soften up” the rock in front of a rotary drill bit bysmall/micro-cracks in the rock lattice when expected increasedresistance from rock formation 40 to the drill bit teeth/cutters ispre-detected by detecting device 31, exceeds a given limit/value whiledrilling are also illustrated in FIG. 2, FIG. 3, and FIG. 4. Referencenumbers referred to below, but not shown in FIG. 2, FIG. 3, and FIG. 4,are shown in FIG. 1. FIG. 2 illustrates that the cutter teeth 21 orscrapers 22 of the drill bit 20 is about to hit a hard rock formation40. The detecting device 31, thus engages the switch 35. One or moreshort high voltage electro pulse(s) 41 is/are then generated through therock matrix 40 as illustrated in FIG. 2. After the electro pulse(s), thelattice of the rock formation 40 will contain internalsmall/micro-cracks 45 as illustrated in FIG. 3. The force from the teeth21 or scrapers 22 in the drill bit 20 that is now required to excavatethe rock matrix 40, which now contain small/micro-cracks 45, will beless than the predetermined limit set for the detecting device 31. Thesmall/micro-cracked rock lattice 40 will then be excavated as shown inFIG. 4. When the rock lattice with small/micro-cracks has beenexcavated, the electro pulse process may be reinitiated as the teeth 21or scrapers 22 of the drill bit 20 again are about to hit hard rockformation 40 having a resistance above the predetermined value for theformation to be drilled, as illustrated in FIG. 2. During drilling, highvoltage electro pulses may be generated when pre-detected resistancevalues are above the predetermined limits sensed by detecting device 31.This enables efficient drilling with approximately constant rotationalspeed on the drill bit as the resistance experienced by the drill bitdue to friction between the rotating drill bit and the rock formation ismaintained about a same level throughout the drilling process. How oftenthe high voltage electro pulses are generated, depends e.g. on thepredetermined limits set from the drilling equipment and the propertiesof the rock formation.

The amount of electric current needed for drilling a given amount ofhard rock formation as illustrated in FIG. 2 is depending on theproperties of the rock lattice 40. This amount of electric current isestimated to be considerably less than when drilled by electro pulsedrilling as the main drilling method. The use of electro pulsing in thepresent invention only provides formation of small/micro-cracks in arock lattice upon mechanical drilling/excavation and it does not requirelarge cracks that chip off large bits from the rock lattice as requiredwhen using single electro pulse methods for excavating a borehole. Theamount of electric current is estimated to be in the order of 50% orless. A considerable reduction of the amount of electric current/energymay e.g. enable drilling assemblies having internal energy supplieslasting the entire life time of the drill bit itself.

The principle of the present invention may in a similar way be used inany other system for rotary drilling of inhomogeneous and/or hard rockformations. An example, but not restricted thereto, is for tunneling asshown in FIG. 5. Electrodes may be incorporated in cutter disks 51and/or scrapers 52, as separate electrodes 53, and/or as the drill headof 50 itself, in a rotary tunnel excavator 50. The electro pulses areonly triggered when needed due to the detected resistance (may bemeasured or pre-detected) from the rock formation on the cutter disks 51or scrapers 52 at rotation of the rotary tunnel excavator 50. Electropulsing is provided by the electrodes in a similar manner as explainedfor the embodiment shown in FIG. 1, and illustrated in FIGS. 2, 3 and 4.Small/micro-cracks are thus formed in the rock formation prior toexcavation by the cutter disks 51 or scrapers 52.

The present invention concerns also a method for rotary drilling of aborehole 12 in inhomogeneous or hard rock formations. A rotary drill bit20 us used for rotary drilling of the borehole 12. During drilling aresistance on the drill bit from the formation is detected. When thedetected resistance exceeds a predetermined value, at least one highvoltage electro pulse 41 is generated between the at least twoelectrodes creating small/micro-cracks 45 in the rock formation 40.Monitoring of the resistance during drilling may be performedcontinuously, almost continuously or at specific intervals.

The a least two electrodes 21, 22, 23, 51, 52, 53 may be incorporated inthe drill bit 20. One of the electrodes may also be the drill bititself. The rotary drill bit can also be a rotary tunnel excavator 50 asshown in FIG. 5. Detecting the resistance may be performed by detectinga torque on the drill bit 20 from the rock formation. In an alternativeembodiment, detecting the resistance may be performed by a radar orsonar pre-detecting the resistance on the drill bit 20 from the rockformation 40.

The drill bit, which is rotating at its intended rotational speed duringthe measurement and high voltage pulse generation procedure, excavatesthe small/micro-cracked rock lattice or matrix 40. When the measured orpre-detected resistance again exceeds the predetermined value, thisagain results in the generation of the at least one high voltage electropulse 41. Monitoring of the resistance during drilling and pulsing highvoltage electro pulses 41 only when needed, ensures that the drill bitexperiences about the same resistance/force from the formation at alltimes during drilling. This considerably improves and simplifiesdrilling in inhomogeneous and/or hard rock formations, at the same timeas the energy consumption is considerably reduced.

The present invention may be used by any rotary drilling method ininhomogeneous and hard rock formations. Primary use is drilling ofwellbores for oil, gas, mining and geothermal exploration andexcavation, and any tunneling with rotary equipment for mining or anyinfrastructures, like but not restricted to, for hydro power, electriccables, roads, train, and water.

Although a variety of examples and other information was used to explainaspects within the scope of the appended claims, no limitation of theclaims should be implied based on particular features or arrangements insuch examples, as one of ordinary skill would be able to use theseexamples to derive a wide variety of implementations. Further andalthough some subject matter may have been described in languagespecific to examples of structural features and/or method steps, it isto be understood that the subject matter defined in the appended claimsis not necessarily limited to these described features or acts. Forexample, such functionality can be distributed differently or performedin components other than those identified herein. Rather, the describedfeatures and steps are disclosed as examples of components of systemsand methods within the scope of the appended claims.

1-25. (canceled)
 26. A rotary mechanical drilling system for mechanicaldrilling of a borehole in inhomogeneous and/or hard rock formations, thesystem comprising: a rotary drill bit, and at least two electrodesarranged for pulsing between said electrodes of at least one highvoltage electro pulse, wherein said at least one high voltage electropulse is generated in response to a detected resistance on the drill bitfrom a rock formation in front of the drill bit while drilling, in orderto create small-cracks/micro-cracks in the rock formation, wherein saidat least one high voltage electro pulse is generated only when thedetected resistance on the drill bit exceeds a predetermined limit underdrilling.
 27. The rotary drilling system according to claim 26, furthercomprising at least one sensor arranged in the drill bit for detectingthe resistance on the drill bit from the rock formation.
 28. The rotarydrilling system according to claim 26, further comprising at least onesensor arranged in the drilling system for detecting the resistance onthe drill bit from the rock formation.
 29. The rotary drilling systemaccording to claim 26, further comprising a radar or sonar forpre-detecting the resistance on the drill bit from the rock formation.30. The rotary drilling system according to claim 26, wherein thedetected resistance is in the form of a detected torque on the drill bitfrom the rock formation.
 31. The rotary drilling system according toclaim 30, wherein said at least one high voltage electro pulse isgenerated when the detected torque on the drill bit exceeds apredetermined torque limit under rotary drilling.
 32. The rotarydrilling system according to claim 26, wherein at least two electrodesare incorporated in the drill bit.
 33. The rotary drilling systemaccording to claim 26, further comprising an electro pulse deviceconnected to said electrodes and adapted for generating said at leastone high voltage electro pulse.
 34. The rotary drilling system accordingto claim 26, further comprising a torque sensing device arranged fordetecting the drill bit torque from the rock formation in front of thedrill bit under drilling.
 35. The rotary drilling system according toclaim 26, further comprising an electronic switch for electro pulsing.36. The rotary drilling system according to claim 26, wherein theelectro pulse device comprises at least one of: a capacitor, a rectifierand a transformer of low to high voltage and/or current.
 37. The rotarydrilling system according to claim 26, wherein the system furthercomprises an electrical source comprising at least one of: a batterypack or batteries, a mud driven generator, a water driven generator, acompressed air driven generator, a rotary drill string driven generator,and cables to surface.
 38. The rotary drilling system according to claim26, further comprising a drilling assembly.
 39. The rotary drillingsystem according to claim 38, wherein the rotary drill bit is arrangedin the drilling assembly.
 40. The rotary drilling system according toclaim 38, wherein the electro pulse device is arranged in the drillingassembly.
 41. The rotary drilling system according to claim 26, whereinthe drill bit comprises at least one of: teeth, scrapers and cutterdisks, and wherein said at least two electrodes are incorporated in atleast one of: the drill bit itself, the teeth, the scrapers and thecutter disks.
 42. The rotary drilling system according to claim 26,wherein the system is adapted for tunneling, and the drill bit is arotary tunnel excavator.
 43. A method for rotary mechanical drilling ofa borehole in inhomogeneous and/or hard rock formations, comprising:rotary mechanical drilling a borehole using a rotary drill bit, pulsingbetween at least two electrodes of at least one high voltage electropulse, wherein said at least one high voltage electro pulse is generatedin response to a detected resistance on the drill bit from a rockformation in front of the drill bit while rotating, creating smallcracks/micro-cracks in the rock formation, wherein said at least onehigh voltage electro pulse is generated only when the detectedresistance on the drill bit exceeds a predetermined limit under rotarydrilling.
 44. Method according to claim 43, further comprising detectingthe resistance by detecting a torque on the drill bit from the rockformation.
 45. Method according to claim 43, further comprisingdetecting by using a radar or sonar for pre-detecting the resistance onthe drill bit from the rock formation.
 46. The rotary drilling methodaccording to claim 43, wherein at least two electrodes are incorporatedin the drill bit.
 47. The rotary drilling method according to claim 43,further comprising excavating the micro-cracked rock lattice or matrix.48. The rotary drilling method according to claim 43, wherein the methodis adapted for tunneling, and the drill bit is a rotary tunnelexcavator.